The present invention relates to automatic brake control systems for vehicles, and is particularly applicable to brake systems for automobiles.
This invention relates move particularly to a failure detecting system for detecting failures of a vehicle velocity detector and a wheel velocity detector forming part of a vehicle brake controlling device which performs control over the vehicle brake system on the basis of a comparison of the vehicle velocity signal and wheel velocity signal derived from the vehicle velocity detector and the wheel velocity detector provided in the vehicle.
In automatic brake control systems for automobiles, there has heretofore been provided an anti-skid controller in which the slip state of the wheel during braking is detected by making use of the vehicle velocity signal and the wheel velocity signal derived from a vehicle velocity detector and a wheel velocity detector, and a braking device is controlled on the basis of a comparison of these detected signals whereby the wheel is prevented from being braked too severely or locked causing the vehicle to undergo a sideslip or a so-called skid by temporarily releasing the brakes when such conditions are detected.
The anti-skid controller performs a comparison of the output signals from the vehicle velocity detector and the wheel velocity detector in comparison means during braking and determines from this comparison whether the wheel velocity has lowered beyond a certain predetermined proportion relative to the vehicle velocity. When this condition is detected, it is assumed that the wheel slips, and therefore, an output signal "0" is generated from the comparison means to an AND gate which serves to prevent the actuating signal for the braking system from being delivered by the AND gate even though the braking signal is being received at that time. Thus, the braking force on the vehicle is immediately released.
With such a controller, however, where a failure or other malfunction occurs in the vehicle velocity detector or the wheel velocity detector, a dangerous condition is established. When the vehicle velocity detector fails in some way so that the vehicle velocity signal is not provided or its output level or output frequency is extremely low as compared with a normal value, or where the wheel velocity detector fails so that the wheel velocity signal is abnormally high as compared with a normal value, the level of frequency of the wheel velocity signal exceeds that of the vehicle velocity signal even when the slipping occurs during braking. The output signal of the comparison means accordingly holds a state "1", so that the antiskid controller does not function and the dangerous operating condition of the vehicle is not corrected.
Conversely, where the wheel velocity detector fails in such a way that the wheel velocity signal is not provided or its output level or output frequency is extremely low as compared with a normal value, or where the vehicle velocity detector fails so that the output level or output frequency of the vehicle velocity signal is abnormally high as compared with a normal value, the output signal of the comparison means becomes "0" notwithstanding the fact that no slipping is actually occurring. Thus, during braking under these conditions, the braking device becomes quite inactive, producing a dangerous condition.